Various cordless electrical devices may be powered by a battery pack. Examples of such cordless electrical devices include, but are not limited to, laptop computers, cell phones, personal digital assistants, and power tools such as a drill. The battery pack may include a plurality of battery cells and power switches to allow the battery cells to either supply current (discharge operation mode) or to be charged (charge operation mode). The battery pack may also include battery monitoring circuitry powered by the battery cells to perform of number of tasks to maintain a safe and desired use of the battery cells. A large variety of such battery state monitoring circuitries have been developed to accommodate differing power management topologies.
During different times, the battery pack may be in a stand-by-state. During the stand-by-state, the battery cells neither supply a current to the load nor are connected to a charging power source. During this stand-by-state, the battery monitoring circuitry may also be in a low power state. To sense an end to the stand-by-state, conventional monitoring circuitries utilize internal components in conjunction with a sense resistor in series with the battery cells. These components may include a differential sense amplifier amplifying the voltage drop across the current sense resistor, a voltage translator that receives an output of the differential sense amplifier, and a comparator that receives an output of the voltage translator and compares that with a threshold level in order to determine the end of the stand-by-state.
In this conventional approach, and in many other similar approaches, the corresponding components of the battery state monitoring circuitry (e.g., the sense amplifier, voltage translator, and comparator as well as associated biasing and reference circuitry) consume excessive power in the stand-by-state that adversely impacts battery pack performance. For example, when the stand-by-state extends over a long period of time such as hundreds of hours, the power consumption of the battery state monitoring circuitry itself may cause a significant battery discharge. In addition, in this conventional approach the ON resistance of the discharge power switch of the battery pack remains fully ON having the same ON resistance whether the battery pack is in the stand-by-state or not.
Accordingly, there is a need for battery monitoring circuitry with relatively low power consumption during the stand-by-state of the battery pack.